This invention relates to a low temperature method and apparatus for making optical fibers, and more particularly to such a method and apparatus which are useful in the manufacture of low loss optical waveguide fibers capable of propagating energy in the infrared region of the spectrum.
The overloaded condition of communication systems in the 1960's motivated a search for higher capacity transmission media. Although glass optical fibers were known, the attenuation thereof was too high for use in such systems. Improved processing techniques such as that disclosed in U.S. Pat. No. 3,711,262 have resulted in optical waveguide fibers having attenuation levels sufficiently low that they are now widely used in telecommunication systems. Such progress has been made in silica-based fibers that fibers of this type have been produced having attenuations less than 0.5 dB/km at 1.55 .mu.m. It has been reported that the ultimate loss for silica glass fibers might be 0.18 dB/km, which is limited by intrinsic loss due to infrared absorption bands and Rayleigh scattering.
In order to achieve even lower loss transmission, materials capable of transmission in the infrared region of the spectrum are being investigated. The publication, J. R. Gannon, "Optical Fiber Materials for Operating Wavelengths Longer Than 2 .mu.m", Journal of Non-Crystalline Solids, vol. 42 (1980), pp. 239-246, sets forth calculated attenuation minima for a number of potential materials. It is speculated that ZnCl.sub.2 and BeF.sub.2 should achieve attenuation minima of 0.001 dB/km at 3.7 and 2.1 .mu.m, respectively.
U.S. Pat. No. 4,188,089 teaches a CVD technique for forming fibers having light-conducting regions formed of selected non-oxide compositions. A carrier gas such as chlorine or any other halogen or an inert gas such as He, Ar, or Kr entrains vapors of the reactant gases. All such vapors are premixed and delivered to the end of a substrate tube through which they flow unreacted until they reach the moving hot zone. Upon reaching the region of elevated temperature, a reaction takes place which causes a vitreous layer to be deposited within the substrate tube. This layer is composed predominantly of anions S, Se and/or Te. Cations are Ge, Si, P, B, As, Sb and/or Ti. The light transmitting core of the resultant optical waveguide fiber cannot include certain elements such as beryllium, zinc, aluminum, lead and the like which do not form volatile halides.
Fluoride glass fibers formed by a conventional rod drawing technique are disclosed in the publication, S. Mitachi et al., "Fluoride Glass Fiber for Infrared Transmission", Japanese Journal of Applied Physics, vol. 19, No. 6, June, 1980, pp. L313-L314. Mixtures of GdF.sub.3, BaF.sub.2 and ZrF.sub.4 were melted at 800.degree. C. in a gold crucible, and glass rods were prepared by pouring the melts into brass molds. The glass rods were drawn into fibers by a conventional fiber fabrication technique. Losses as low as 480 dB/km were measured at 3.39 .mu.m for fibers several meters long.
U.S. Pat. No. 4,189,208 teaches a method of forming an optical fiber having a core of ZnCl.sub.2 by drawing the fiber from a melt. Because of the impurities which are introduced into the fiber by forming it in this manner, such a technique has not been capable of providing fibers having losses as low as those which are formed by vapor deposition techniques.
U.S. Pat. No. 3,722,981 teaches a method of drawing low-melting fluoride glasses from a melt. Although it is estimated that the Rayleigh scattering loss of such fibers is about 1 dB/km, the impurity absorption losses of fibers formed by this technique are unacceptedly high. It is stated in the publication, Y. Ohishi et al., "Impurity Absorption Loss due to Rare Earth Elements in a Fluoride Glass", Japanese Journal of Applied Physics, vol. 20, No. 3, March, 1981, pp. L191-L193, that rare earth impurity concentration in fluoride optical fibers should be decreased to less than 0.1 ppbw in order to make the absorption loss 0.001 dB/km in the 3-4 .mu.m wavelength range.
It is therefore an object of the present invention to provide a method and apparatus for forming by a vapor deposition process optical waveguide fibers which are suitable for transmitting energy in the infrared region of the spectrum.
Another object is to provide a method and apparatus for delivering highly reactive reactants to the deposition region of a substrate tube.